Week 9 (proteins III) Flashcards
What are heam groups?
Both myoglobin and haemoglobin contain haem prosthetic groups.
- Heme is composed of a ringlike organic compound known as a porphyrin, to which an iron atom is attached.
- It is the iron atom that reversibly binds oxygen as the blood travels between the lungs and the tissues.
- Thus both myoglobin and haemoglobin exist in a deoxygenated and an oxygenated form.
- The haem group is planar.
What is the structure of myoglobin?
- 154 amino acid residues
- 75% of amino acids in α-helices
- 8 major helical regions
- 1 Haem group
- Compact tertiary structure
- Non-polar or hydrophobic groups in the core
- Hydrophilic/polar groups on the outside.
- Protein is water soluble
- Monomeric
What is the structure of haemoglobin?
- Haemoglobin is a tetramer
- Four heme groups surrounding a globin group, forming a tetrahedral structure
- 2 α subunits and 2 β subunits with 146 amino acid residues in each (these polypeptide chains are linkedby non covalent interactions)
- Each subunit is folded very similarly to myoglobin.
Note haem groups are far apart from each other*
Does myoglobin or haemoglobin have a higher affinity for oxygen?
Myoglobin
Although its heme group is identical to those in Hb, Mb has a higher affinity for oxygen than does hemoglobin. This difference is related to its different role: whereas hemoglobin transports oxygen, myoglobin’s function is to store oxygen.
Describe the Sigmoid binding curve
-The sigmoidal or S-shape curve is critical for release of oxygen when it gets to tissues
What does affinity refer to?
Affinity refers to how tightly two molecules interact or bind
What does Oxygen binding induce?
A conformational change: Deoxyhemoglobin to oxyhemoglobin
Describe Cooperativity between subunits
- The deoxygenated form is referred to as the T (tense) state.
- The oxygenated form is referred to as the R (relaxed) state.
- In the R state the affinity for oxygen is markedly increased→binding of oxygen to one subunits increases the affinity at other subunits
What does binding of oxygen to one subunit cause?
Binding of oxygen to one subunit increases the affinity for oxygen at neighbouring subunits
What is Allostery?
The binding of a ligand to one site on the protein which affects the binding properties of another site on the same protein.
What is Cooperativity?
- Can occur in multimeric assemblies.
- The binding of one ligand affects the affinities of any remaining unfilled ligand binding sites.
- Gives characteristic sigmoidal binding curve.
Binding of oxygen to haemoglobin is
both allosteric and cooperative
Is the oxygen affinity of whole blood is lower than that of free haemoglobin?
Yes
Does Purified haemoglobin have a higher affinity for oxygen?
Yes
What is 2,3-Bisphosphoglycerate?
- 2,3-Bisphosphoglycerate (BPG) is present in red blood cells at approximately the same concentration as haemoglobin (≈2mM)
- It is critical for haemoglobin to be an efficient oxygen transporter
Binding of BPG to deoxyhaemoglobin
- BPG only binds to deoxygenated haemoglobin
- One molecule of BPG binds per haemoglobin tetramer
Explain Electrostatic interactions between BPG and haemoglobin
BPG binding cross links the two β subunits by electrostatic interactions
- BPG stabilises the deoxygenated form (which decreases the affinity for oxygen and allows it to come off in tissues)
- BPG is an allosteric effector
Foetal haemoglobin binds BPG less tightly than adult haemoglobin
Foetal haemoglobin has a different subunit composition, α2γ2
- γ chain is 72% identical to β chain
- One big difference is substitution of His143 with a serine.
- His143 positive charge is important in binding BPG
- Foetal haemoglobin binds BPG with a lower affinity than adult haemoglobin does.
- Lower affinity for BPG – higher affinity for oxygen
Explain Sickle cell anaemia
- Inherited disease which becomes apparent in homozygotes
- On deoxygenation the red blood cells sickle.
- Sickle cells are fragile and rupture easily
- Causes anaemia
- Life-threatening and painful – capillaries can often become blocked by the long abnormal cells
Substitution of a glutamate for a valine – Haemoglobin S
A single nucleotide mutation (A-T) in the gene encoding Haemoglobin β chain Results in substitution Glu6 for Valine.
Valine is hydrophobic and wants to bury away from water.
In the deoxygenated form it can interact with Phe85 and Val88 on β chain of a neighbouring molecule
Deoxygenated haemoglobin S forms polymers
- Insoluble fibres of haemoglobin form which cause the red blood cells to sickle due to the amino acid change which causes the hemoglobin to associate with other hemoglobin molecules
- Fiber formation only occurs in the deoxy or T-state.
- Because of the structural change to form the T-state, a different region of the protein exposes a hydrophobic surface area.
- The area containing the mutated amino acid residue and the area exposed by forming the T-state associate together to form the fibers
Haemoglobin is affected by both pH and CO2: the Bohr effect
In tissues undergoing respiration (eg. muscle) large amounts of CO2 and H+ are generated.
- To release oxygen where need is greatest , haemoglobin has evolved to respond to this.
- Increase [H+] decrease pH •CO2 + H2O H+ + HCO3- Catalysed by carbonic anhydrase which is particularly abundant in red blood cells
Bohr effect part 1: stabilisation of the protonated form of His 146
- Lower pH His 146 is protonated
- Electrostatic interactions stabilise the deoxygenated T state of haemoglobin
Bohr effect part 2:
CO2 also directly affects haemoglobin
Bohr effect part 2: carbamate formation
- CO2 can reacts with the N-terminal amino groups to form carbamate groups
- Carbamate groups are negatively charged
- These termini lie at the interface between subunits
- The negative charge forms electrostatic interactions which stabilise the deoxygenated T state of haemoglobin – decreased affinity for oxygen
Explain why carbon monoxide has a very high affinity for haemoglobin
Colourless, odourless gas
- Binds to haemoglobin at the same site as oxygen
- Affinity for CO approx. 200 times higher than affinity for oxygen
- So even small amounts in the blood will displace oxygen
- CO bound to just one subunit will shift haemoglobin to the high affinity R state, so haemoglobin can still bind oxygen, but can’t release it to the tissues – severe hypoxia Stabilises oxygenated form so no oxygen not released to the tissues
Models to explain cooperativity: the concerted model
- Used to explain the cooperativity in oxygen binding as well as the transitions of proteins made up of identical subunits.
- It focuses on the two states of the Hemoglobin; the T and R states.
- The T state of the hemoglobin is more tense as it is in the deoxyhemoglobin form while the R state of the hemoglobin is more relaxed as it is in the oxyhemglobin form.
- The T state is constrained due to the subunit-subunit interactions while the R state is more flexible due to the ability of oxygen binding.
- The binding of oxygen at one site increases the binding affinity in other active sites.
- Thus in the concerted model of the hemoglobin, it shows that the one oxygen binding to an active site will increase the probability of other oxygen binding to the other active sites
Models to explain cooperataivity: the sequential model
- This model explains the cooperativity involved in the binding of oxygen.
- This model follows the concept that after binding occurs at one site in the active site, the binding affinity in the other sites around the protein will increase as well.
- Hence, the plot of substrate concentration versus reaction rate is of a sigmoidal shape
